Four people were killed and over four others wounded on Sunday when a mortar shell slammed the fairgrounds in the capital Damascus, where a premier trade fair is being held, a well-informed source told Xinhua.
"Their direct support - politically, economically and militarily - has made possible advances on the battlefield and reduced the losses and burdens of war", Assad added.
The government in Damascus touts the reopening of the gathering after a six-year delay as a sign of life getting back to normal in Syria.
The UK-based Syrian Observatory for Human Rights also reported an explosion in the city saying it was likely caused by a vehicle blast and said there were several injuries, with some in critical condition.
Businessmen from countries like Libya, Iraq and Kuwait, as well as several delegations from foreign governments are attending the event.
"We want this fair to signal the start of (the country's) reconstruction", Kartally told AFP earlier this week. The first time it was held was in 1954, and the last time in 2011.
While Damascus has been insulated from much of the worst violence of the country's war, several key rebel enclaves remain in the Eastern Ghouta region outside the city.
The news was broken on Sunday by Syrian state television, but no details were immediately reported.
Since then, the country has descended into a bloody civil war that has killed more than 330,000 people, displaced millions and devastated the economy.